花花酱 LeetCode 2170. Minimum Operations to Make the Array Alternating

You are given a 0-indexed array nums consisting of n positive integers.

The array nums is called alternating if:

• nums[i - 2] == nums[i], where 2 <= i <= n - 1.
• nums[i - 1] != nums[i], where 1 <= i <= n - 1.

In one operation, you can choose an index i and change nums[i] into any positive integer.

Return the minimum number of operations required to make the array alternating.

Example 1:

Input: nums = [3,1,3,2,4,3]
Output: 3
Explanation:
One way to make the array alternating is by converting it to [3,1,3,1,3,1].
The number of operations required in this case is 3.
It can be proven that it is not possible to make the array alternating in less than 3 operations. 

Example 2:

Input: nums = [1,2,2,2,2]
Output: 2
Explanation:
One way to make the array alternating is by converting it to [1,2,1,2,1].
The number of operations required in this case is 2.
Note that the array cannot be converted to [2,2,2,2,2] because in this case nums[0] == nums[1] which violates the conditions of an alternating array.

Constraints:

• 1 <= nums.length <= 105
• 1 <= nums[i] <= 105

Solution: Greedy

Count and sort the frequency of numbers at odd and even positions.

Case 1: top frequency numbers are different, change the rest of numbers to them respectively.
Case 2: top frequency numbers are the same, compare top 1 odd + top 2 even vs top 2 even + top 1 odd.

Time complexity: O(nlogn)
Space complexity: O(n)